This invention relates to a circuit for use with a voltage regulator. Such circuits find use with, for example, voltage regulators for use in automotive applications, e.g. for regulating the automotive battery voltage applied to an electronic module such as an ignition or anti-lock braking control module.
In a typical known circuit for use with a voltage regulator in automotive applications the circuit comprises a bipolar pnp transistor having its emitter connected at the circuit's input to battery voltage via a diode, and its collector connected to provide a regulated voltage at the circuit's output. The transistor's collector is connected to provide feedback to a regulator which produces a control signal applied to the transistor's base. Input and output charge storage capacitors are connected between earth and, respectively, the transistor's emitter and collector.
The diode protects the transistor and the input capacitor from being subjected to a negative voltage which may be presented at the circuit's input. The input capacitor (which, since it is protected by the diode, can be electrolytic) is charged to a voltage close to the battery voltage, and so prevents brief negative voltage transients at the circuit input from propagating through to the output capacitor (which can also be electrolytic). However, the presence of the diode introduces an additional voltage drop between the circuit's input and output which the battery voltage must exceed for satisfactory operation. In a typical such circuit the battery voltage must exceed the regulated voltage by at least 0.9 V for satisfactory normal operation.
If the protection diode were removed, operation would be provided at a lower battery voltage. However, in this case the input capacitor could be exposed to negative voltage presented at the circuit input, and so would have to be non-polar, in which case it would typically be of much smaller value than the output capacitor (which can still be electrolytic) and so could not adequately buffer the output capacitor against negative transients at the circuit input.